Distribution of mesotocin-immunoreactive neurons in the brain of the male native Thai chicken

Mesotocin (MT), a homolog of oxytocin (OT) in mammals, is a nonapeptide neurohypophysial hormone that is mainly synthesized in specific neuronal groups within the hypothalamus and released from the posterior pituitary gland in amphibian, reptilian, and avian species. MT is associated with the neuroendocrine regulation of reproductive cycle and maternal behaviors in female native Thai chickens. Male birds exhibit parental behaviors as well. However, there are limited data regarding the role(s) of the MTergic system in males. Thus, the objective of this study was to elucidate the localization of the MT neuronal groups in the brain of male native Thai chickens. The distributions of MT-immunoreactive (-ir) neurons and fibers in the brain were studied utilizing immunohistochemistry technique. The results revealed that MT-ir neurons and fibers were distributed throughout the brain and extensively in the diencephalon. MT-ir neurons and fibers were predominantly located within the nucleus supraopticus, pars ventralis (SOv), nucleus preopticus medialis (POM), nucleus ventrolateralis thalami (VLT), nucleus paraventricularis magnocellularis (PVN), and regio lateralis hypothalami (LHy), suggesting that MT neurons in these nuclei might be involved in the reproductive activities and/or parental behavior in the male chickens. In addition, the numbers of MT-ir neurons within the SOv and POM were significantly higher than those of the VLT, PVN, and LHy. More importantly, the number of MT-ir neurons in the SOv was high in the male brain when compared with the female brain, indicating that the MTergic system in the SOv might play a significant role in male reproductive activities in this equatorial species.